Electron bombardment melting furnace



July 11, 1967 VON ADRIENNE ETAL 3,330,901

ELECTRON BOMBARDMENT MELTING FURNACE Filed March 25, 1964 INVENTORSMANFRED VON ARDENNE SIEGFRIED SCHILLER United States Patent ELECTRONBOMBARDMENT MELTING FURNACE Manfred von Ardeune and Siegfried Schiller,both of Dresden-Weisser Hirsch, Germany, assignors to VEBLokomotivbau-Elektrotechnische Werke Hans Beimler,

Hennigsdorf, Kreis Oranienburg, Germany Filed Mar. 25, 1964, Ser. No.355,999 1 Claim. (Cl. 13-31) The present invention relates to electronbeam melting furnaces and in particular to improvements in the controlof the electron beam.

In general, two methods have been developed for employing electronbombardment for melting materials such as metals. In one method thecathode is situated in the same vacuum chamber as the material to bemelted, i.e., the melt. In the other method the electron gun chamber, inwhich the cathode is situated, is separated from the melting chamber byone or more vacuum separating chambers. The advantage which the firstmethod presents lies in its structural simplicity. However, the gaseswhich evolve from the melting materials tend to produce undesirabledischarges which may damage or destroy elements of the electron beamgenerator. The discharges result because the gas is generated within thechamber which also includes the high intensity electric field necessaryto generate the electron beam. Thus, additional pumping apparatus mustbe provided during the melting operation to keep the pressure below thedischarge threshold.

The second method obviates the disadvantage noted above by isolating thebeam developing apparatus from the melting section by providing aplurality of chambers between the gun chamber and the melting chamber.Thus, the melting zone may be situated in a field free region.

However, the second method also presents operating disadvantages. Thus,in the second method it has been found diffic-ult to guide highintensity electron beams of, e.g., 7 to 60 a. at 30 kv. beam voltage.The inherent space charge of the beam, i.e. the fields caused by thecharge in the beam itself, produces a significant increase in thediameter of the beam. The diverging efiect of the space charge tends tomake focusing of the beam more difficult. It was therefore necessary tolimit the amount of current directed to the melting chamber to reducethe diverging effect of the space charge. Thus the space chargenecessitated a reduction in the intensity of the electron beam andthereby reduced the effectiveness of the unit.

In the instance where the originally parallel beam has a circularcross-section the following formula is applicable:

wherein: X=space charge spread, r=radius of the cross section, r=initial radius of the beam cross-section, r final radius of the beamcross-Section, j initial density of the electron current in ascmra.=ampere, cm.=centimeter, U accelerating voltage, v.=bolt and L=lengthof the beam in m.=meters. Under normal conditions, in electronbombardment melting furnaces, the value i 2 a. cmf wherein a. cm:-=amperes/cm. and U 30 kv., X-3;8 when L-0.2 meter. Similar results wereobtained for 'bandfor-m beams. Such a beam spread is 3,33%,9hl PatentedJuly 11, 1967 inadmissible in practice because of resulting excessivelyhigh flow resistance which would demand inordinately large vacuum pumps.(In common practice the beam length exceeds 0.2 m. where no focusingmeans are used.) These problems may be solved by providing magneticlenses at suitably short intervals. However, in electron bombardmentfurnaces where magnetic lenses must be employed at short intervalsinsufficient room is left for the evacuation flange. In addition, theextra magnetic lenses add considerable cost to the device.

Auxiliary focusing may be obtained with thread beam formations, e.g., byover compensation of the space charge. This process may be utilized inthe pressure range of approximately 10 millimeters of mercury and withcurrents not in excess of 1 ma. However, in electron bombardmentfurnaces where the current rating is approximately 7 to 60 a. and withaccelerating voltages of, e.g., 20 to 30 kv., it is impossible to applysuch a method since at those high pressures high tension flashoverswould take place and gas dispersion would become undesirably high.

In electron gun bombardment devices having limited output electronbeams, a space charge compensating effect may be achieved by ionformation in the always present residual gas. However, with electronguns having kw. outputs and higher, utilization of the space chargecompensation on residual gas does not provide satisfactory anddependable results. Furthermore, where ions are utilized to neutraliZethe electron space charge a means must be provided to prevent the entryof ions into the electron gun chamber, because the presence of ions inthe gun chamber tends to cause high voltage flashovers therein.

It is therefore an object of the present invention to provide improvedspace charge compensation means in an electron gun apparatus.

It is another object of this invention to provide space chargecompensation of electron beams having a relatively high output.

It is still another object of the present invention to provide animproved electron beam melting furnace.

The apparatus as disclosed herein includes an electron gun chamber forproducing a high output electron beam. The beam is directed through acylinder having a passage down into a melting chamber where the electronbeam impinges upon the material to be melted. A magnetic lens at eachend of the cylinder is employed to provide the requisite beam focusing.An ion trap electrode is positioned proximate the anode of the electrongun at the top of the passage within the cylinder. A separating chamberand -a high pressure chamber are provided intermediate the cylinder endsand are connected to the passage which runs along the longitudinal axisof the cylinder. A Valve provides control of the quantity of fluidtransmitted to the high pressure chamber, through a suitable conduit. Apair of electrodes are positioned within the passage on either side ofthe high pressure chamber. The electrodes serve to pull ions, which havebeen formed in the high pressure chamber, into the electron beam therebydiminishing the effect of the space charge and limiting the divergenceof the electron beam. Suitable pumps and fluid connecting lines areprovided to draw the requisite amount of fluid from within the gun,separating, high pressure and melting chambers.

Other and further objects of this invention will be apparent from thefollowing description and claim and may be understood by reference tothe accompanying drawing, which by way of illustration shows preferredembodiments of the invention and What is now considered to be the bestmode of applying the principles thereof.

In the drawing,

The single figure is a diagrammatic elevational crosssectional view of aheavy duty electron bombardment melting furnace.

The furnace includes an upper cylindrical housing 19 having a flange 12and radial exit ducts 13 and 14 which may be secured to a vacuum pump(e.g. a diffusion pump) to evacuate the housing. A cylindricalinsulating member 16 is mounted vertically on the upper horizontalflange 12. A flat ring 17 rests on the upper surface of insulatingmember 16 and supports an inverted hat shaped focusing electrode 18.Focusing electrode 18 includes a bottom conically shaped opening 20through which the beam of electrons passes. A round cover plate 22 issecured to the upper surface of the flange of electrode 18.

A supporting disc 24 is conventionally secured to the center of thelower surface of cover 22 and supports a pair of cylindrical members 26and 28 between which a second horizontal disc 30 is secured. A beamgenerating cathode 32 is held in the lower surface of cylinder 28 andconcentrically disposed with respect to opening 20.

A generally conically shaped anode 33 is concentrically disposed withrespect to cathode 32 beneath focusing electrode 18. Anode 33 isphysically supported above a magnetic lens which comprises coil 34 andcore member 36 secured to housing in any desired manner. A high voltagemay be applied to a pair of terminals 38 and 40 coupled to cathode 32and anode 33, respectively, to control the flow of electrons through theanode.

A concentric electrode 42 is placed in the lower portion of anode 33within the upper portion of core 36. Electrode 42 is supported by meansof an insulating cylinder 44- situated between appropriate flanges onthe electrode and an internal ledge of core 36. The potential onelectrode 44 is such as to prevent ions from entering the electron gunchamber 11. The space 46 beneath electrode 42 is a part of the electronpath and leads into the separating chamber 48 at the bottom of housing10.

A vertical cylindrical support member 52 is positioned between a lowerhousing 53 and upper housing 10. An electrode 50 is contained withincylinder 52 and supported thereon by means of an insulating cylinder 54.Electrons pass through electrode 50 and aperture 55 into a high pressurechamber 56 in the upper portion of housing 53. The purpose of electrode50 is to draw ions away from chamber 56. Chamber 56 includes a flange 58for connection to a vacuum pump and a gas inlet valve 60.

A second magnetic lens is located in the bottom portion of chamber 56and comprises a coil 62 and core 64 concentrically arranged with respectto the path of the electron flow and including an electron path 66 and alower electrode 68 supported as in the case of electrodes 50 by means ofan insulating cylinder 70. The purpose of electrode 68 is to draw ionsaway from chamber 56.

The assembly, of course, includes a melting chamber 72 in which the barto be melted 74 is located above a liquid material 76 in a water cooledcopper crystalizer 78.

Electrical means (not shown) serve to provide the electromagnetic fieldsnecessary for the proper functioning of the electrodes.

Turning now to the operation of the invention, the cathode and anodecooperate to provide an electron beam which is directed down through thecenter of the furnace to finally impinge upon bar 74. High pressurechamber 56, through which the beam passes, serves as an ion source. Thepressure in chamber 56 is normally in the vicinity of 310* to 10-millimeters of mercury column, depending upon the quantity of ions to beproduced. In low energy electron beams, the potential drop produced bythe electron beam charge and the diffusion produced by the passage ofthe beam through the fluid of the passage suflices to draw off asuflicient quantity of ions to limit the diverging effect of the spacecharge on the electron beams. However, as noted above, in high outputheavy duty electron bombardment furnaces, towards which this inventionis directed, the quantity of ions drawn will not sufficiently neutralizethe space charge. Electrodes 50 and 68 are provided to pull additionalions from chamber 56 into the electron beam. Since the electrodes arepositioned one on each side of the chamber the ions are pulled both inthe direction of the beam as well as in the opposite direction.

If additional ions are required the ion stream may be intensified byauxiliary fields generated by electrode 56 in combination withseparating chamber 48. The slow electrons generated at the same time inchamber 56 travel toward the walls of passage 55. The ions are drawntowards the beam axis as a result of the potential drop between the beamboundary and the beam axis. Thus, relatively few recombinations occur onthe walls of passage 55. Similar conditions prevail in the case ofrectangular beam cross-sections.

The pressure in chamber 56 may be controlled by adjusting the gas inletvalve 60. If the vacuum in melting chamber 72 is very high (e.g., 10 tol() millimeters of mercury column) more gas must be admitted into highpressure chamber 56 to thereby raise the pressure in the chamber higherthan the pressure in the melting chamber, e.g., 10- millimeters ofmercury column. In practice the pressure in the melting chamber isusually between 10 and 10* millimeters of mercury. Since the flowresistance of a tube is proportional to the root of the molecularweight, it is desirable to use a heavy gas, e.g.,

argon.

The ions penetrated to the gun chamber would cause flashovers andconsiderable damage to the cathode would result. To obviate thisdrawback, ion trap electrode 42 is connected to a power source (notshown), at a potential which will repel the ions penetrated to cathodechamber through passage 46. It should be noted that electrode 42 mayhave positive as well as negative potential relative to the anode. Thus,the ions in the gun chamber may be repelled or pulled out of the gunchamber. Because the spacing between the cathode and magnetic lens 34 orelectrode 42 is small (e.g., 50 to millimeters) the space charge spreadis not critical within the gun chamber. Thus, the removal of ions fromthe gun chamber will not effect eflicient operation of the device.However, the distance between magnetic lenses 34 and 62 in electronbombardment melting furnaces such as embodied herein is in the order of700 millimeters. Therefore, in this instance, the space charge spreadbetween the magnetic lenses does become critical. Hence, it is necessaryto produce additional ions to counter-act the diverging effect which thespace charge has on the electron beam.

We have described what we believe to be the best embodiments of ourinvention. We do not wish, however, to be confined to the embodimentsshown, but what we desire to cover by Letters Patent is set forth in thefollowing claim.

What is claimed:

An electron bombardment melting furnace comprising an elongated cylinderhaving a passage extending along the longitudinal axis thereof, an upperhousing encompassing the upper portion of said cylinder, said housingincluding an electron gun chamber, a cathode positioned within saidelectron gun chamber above said cylinder and in alignment with saidcylinder passage, an anode positioned directly above said cylinder andin concentric relation thereto, a focusing electrode positioned betweensaid anode and said cathode, an ion trap electrode concentricallymounted 'within said passage in close proximity to said anode, aseparating chamber, at the lower end of said upper housing, inconnection with said passage, means for removing fluid from saidseparating chamber,

a lower housing encompassing the lower portion of said cylinder, a highpressure chamber in the top of said lower housing, means for introducingfluid into said high pressure chamber, means for removing fluid fromsaid high pressure chamber, a first ion drawing electrode concentricallymounted within said passage between said upper and lower housing fordrawing ions from said high pressure chamber, a second ion drawingelectrode concentrically mounted within said passage beneath said highpressure chamber for drawing ions from said high pressure chamber, aplurality of magnetic lenses mounted concentrically around the outsideof said cylinder and a melting chamber disposed beneath said cylinder,said cathode and anode being adapted, in cooperation with saidelectrodes, chambers and magnifying lenses to produce an electron beamwhich is focused upon an element in said melting chamber.

References Cited UNITED STATES PATENTS JOSEPH V. TRUHE, PrimaryExaminer.

